This invention relates to a drive circuit for giving a very small or fine displacement to an electromagnetic actuator in accordance with a signal from a signal source, while suppressing mechanical resonance. The electromagnetic actuator drive circuit of the invention can be used for driving an electromagnetic actuator used for driving a magnetic head In a video tape recorder or an optical device in a projection image display.
In order to supply an electrical drive signal to the electromagnetic actuator and control, for example, the movement of the magnetic head of a video tape recorder so that the magnetic head can accurately trace the recorded locus of a magnetic tape, there has been considered a damping method of suppressing the mechanical vibration of an actuator by a drive circuit. There is a conventional actuator drive circuit disclosed in Japan Laid-Open Patent Publication No. 64-62813 or in the corresponding U.S. Pat. No. 4,970,611. In another conventional drive circuit, which is described in "Dynamic Tracking System Using Moving Coil Actuators for a Consumer VCR" (IEEE Trans. on Cons., Elec., Vol. 40, No. 4, pp. 969-975, Nov., 1994), there is described a moving-coil type actuator where the video frame frequency is, for example, 30 Hz. In the case of a moving-coil type actuator, with its coil resistance being 31 .OMEGA., and the current-to-displacement conversion sensitivity or gain being 2.1 .mu.m/mA, when the mechanical resonant frequency of the moving part is about 320 Hz, the mechanical vibration can be damped with a Q factor of 4.6 at resonance, if the Q factor is 440 in the current drive of the actuator.
FIG. 15 is a circuit diagram showing an equivalent circuit of an electromagnetic actuator device. In the figure, reference numeral 100 indicates the drive circuit of an actuator 1, reference numeral 2 a resistor equivalent to the resistance value R.sub.c of an electromagnetic coil which constitutes the actuator 1, reference numeral 3 a power supply equivalent to an electromotive force E.sub.a induced in the coil, and reference numeral 4 a magnet placed in the magnetic field that the coil forms. Reference numeral 50 represents an electrical resistance equivalent to a mechanical damping resistance, which has an electrical resistance value R.sub.a. By applying a terminal voltage E and a terminal current I from the drive circuit 100 to the actuator 1, as the drive signal, a desired relative displacement can be given to the actuator 1 (coil) or the magnet 4. However, in order to accurately control the displacement, there is the need to precisely damp the mechanical vibration of the actuator 1 or the magnet 4.
Based on the equivalent circuit diagram, the damping and movement (displacement) of the actuator 1 will hereinafter be examined.
Eq. 1 represents the Laplace transformation of the equation of motion of the actuator 1. ##EQU1## where .omega..sub.a is the mechanical resonant frequency of the moving part of the actuator 1, Q.sub.a is the Q factor of mechanical resonance, G is the current-to-displacement conversion sensitivity of the actuator 1, Y is the displacement of mechanical vibration, and s is the Laplace operator. If the actuator 1 is current-driven, damping is effected with the Q factor which is now equal to Q.sub.a, On the other hand, the relation between the voltage E and the current I of the actuator 1 is expressed by the following Eq. 2: ##EQU2##
Also, if the magnetic flux near the coil is represented by B and the length of the coil is represented by L, the electromotive force E.sub.a induced in the coil becomes as follows. EQU E.sub.a =sYBL (3)
Therefore, the equation of motion of Eq. 1 can be rewritten as the following Eq. 4 by Eqs. 2 and 3. ##EQU3##
When the electromagnetic actuator 1 is voltage-driven at an applied voltage E, the electromotive force E.sub.a is induced in the coil. This electromotive force E.sub.a is fed back to the drive circuit 100, and a damping current E.sub.a /R.sub.c (=sYBL/R.sub.c), which is equivalent to the electromotive force E.sub.a short-circuited by the resistance value R.sub.c of the coil, is produced, whereby a damping force is produced in the electromagnetic actuator 1 by the voltage drive of the actuator 1. The second term on the right-hand side of Eq. 4 indicates the short-circuit damping term, which also becomes the mechanical damping term of speed feedback. Then, the short-circuit damping is replaced with the mechanical damping to obtain the Q factor of the short-circuit damping.
That is, because R.sub.c acts as an electric damping resistor, the short-circuit damping term of the electromagnetic actuator 1 of the second term on the right-hand side of Eq. 4 is transposed to the left-hand side and compared with the mechanical damping term on the left-hand side of Eq. 4. With this, the Q factor Q.sub.c caused by the voltage drive of the actuator 1 is obtained from the following Eq. 5. ##EQU4##
Next, the mechanical damping term of the second term on the left-hand side of Eq. 4 is replaced with the form of the short-circuit damping term and the electrical resistance value in the equivalent circuit of FIG. 15 is obtained.
First, if the numerator and denominator of the mechanical damping term is multiplied by GBL, Eq. 6 is obtained. ##EQU5##
Thus, by rewriting the damping of the mechanical vibration to sYGBL/R.sub.a, the actuator can be regarded as undergoing electric damping at the electrical resistance value R.sub.a. Therefore, the electrical resistance value R.sub.a can be expressed by Q.sub.a which is the Q factor of the mechanical resonance, as follows. EQU R.sub.a =GBL.omega..sub.a Q.sub.a (7)
Referring to this Eq. 7 and the aforementioned Eq. 3, the (numerator/denominator) on the right-hand side of the aforementioned Eq. 6 is represented by E.sub.a /R.sub.a. Therefore, in the equivalent circuit of FIG. 15, the resistor 50 is disposed in parallel with the electromotive force 3.
Also, from Eqs. 5 and 7 the relation of the following Eq. 8 is obtained for the Q factor Q.sub.c in the voltage drive of the electromagnetic actuator 1. ##EQU6##
The aforementioned Eq. 8 indicates that when the electromotive force E.sub.a is fed back, the damping resistance is established by not only R.sub.c but also an arbitrary damping resistance value.
FIG. 16 is a circuit diagram showing an example of a conventional actuator drive circuit.
A signal source 5 in a drive circuit 100 generates a signal voltage E.sub.s, which is input to the negative input terminal of an operational amplifier 7 through an input resistor 6 of resistance value R.sub.1. The positive input terminal of the operational amplifier 7 is connected to ground. The output of the operational amplifier 7 is applied to an actuator 1 through a current detecting resistor 8 of resistance value R. The voltage across the opposite ends of the resistor 8 is amplified by a differential amplifier 9 of amplification degree A, and consequently, the terminal current I of the actuator 1 is fed back to the operational amplifier 7 through a feedback resistor 10 of resistance value R.sub.F and a resistor 11 of resistance value R.sub.2. In this way, current feedback is performed. In addition, the terminal voltage E of the actuator 1 is fed back to the operational amplifier 7 through a capacitor 12 of voltage feedback capacitance C.sub.F and the resistor 11. In this way, voltage feedback is performed.
With the aforementioned arrangement, the actuator 1 gives a very small displacement Y, which is proportional to the signal voltage E.sub.s, to a magnetic head (which is a controlled object) through the magnet 4 of FIG. 15.
The operation of the drive circuit 100 will be examined.
The current I of the actuator 1 gives rise to voltage drop across the current detecting resistor 8. The voltage drop is amplified by the differential amplifier 9 of amplification degree A, and the output of the differential amplifier 9 is negatively fed back to the operational amplifier 7 through the resistors 10 and 11.
When the frequency of the signal voltage E.sub.s of the signal source 5 is low, the capacitor 12 can be considered open. Also, if the resistance value R.sub.F of the resistor 10 is selected to a value which is negligible in comparison with the resistance value R.sub.2 of the resistor 11, the following Eq. 9 is obtained. ##EQU7##
Thus, when the signal frequency of the signal source 5 is low, the drive circuit of FIG. 16 current-drives the actuator 1 and has such a characteristic that the voltage-to-displacement conversion sensitivity (gain) Y of the displacement Y relative to the signal voltage E.sub.s of the input signal is not influenced by the resistance value R.sub.c of the electromagnetic coil of the actuator 1.
When, on the other hand, the signal frequency of the signal source 5 is high, the capacitor 12 can be considered to be short-circuited, and the terminal voltage E of the actuator 1 is negatively fed back to the operational amplifier 7 through the resistor 11 of resistance value R.sub.2. The actuator 1, therefore, is driven by the terminal voltage expressed with the following Eq. 10. ##EQU8##
That is, the actuator 1 is voltage-driven by the negative feedback of the terminal voltage E and, as described above, the Q factor Q.sub.c of the mechanical resonance is reduced to 4.6.
Thus, if current is caused to flow through the electromagnetic actuator 1 to give a desired displacement to a controlled object such as a magnetic head, mechanical vibration will accompany. Damping this vibration is of importance for obtaining a desired displacement and it is possible to perform the damping operation by the drive circuit 100.
However, in order to realize satisfactory tracking and damping operations of a controlled object, a butterworth filter characteristic wherein the Q factor is 0.7 is desirable. In addition, in order to eliminate vibration completely, there has to be realized a critical-damping drive circuit where the Q factor is 0.5. However, in the conventional drive circuit when short-circuit damping is performed by the voltage drive of the actuator, the Q factor which can be realized is only about 4.6 which is about ten times as large as the critical damping.